During the operation of many hydrocarbon wells (i.e. a gas or oil well), the deposition of scale on the formation walls and on production tubing and equipment in contact with well fluids can occur. The severity of the problem is highly dependent on the field operating conditions, which can vary from mild scaling tendencies to the extreme. In the North Sea, typical scales are inorganic salts such as BaSO4, SrSO4, CaSO4 and CaCO3. These salts precipitate as scale when the saturation level of the fluid in which they are present is exceeded. This is clearly detrimental to the operation of the well and the production equipment.
There are two main approaches to preventing or reducing scale formation in hydrocarbon producing systems, namely squeeze treatment and downhole chemical injection (DHCI). Both of these treatment methods involve injection of a scale inhibitor into the hydrocarbon producing system. A major difficulty in preventing scale formation in hydrocarbon producing systems is that they are “open” systems and as such it is a challenge to retain the scale inhibitor within the system, i.e. to prevent it flowing out of the system along with production fluids.
DHCI is most often used to protect the upper part of wells and tubing as well as the downhole safety valve that has scaling potential above the production packer. DHCI is usually used instead of squeeze treatment when a well does not have a high scaling potential in the near wellbore or in cases where scale squeezing would be difficult or costly to perform, e.g. because of tie-in subsea fields.
In a DHCI operation scale inhibitor is injected continuously into the well via a chemical injection line. The injection point is usually upstream of the downhole safety valve. This facilitates dispersion of the scale inhibitor into the production fluids being pumped back to the production platform or refinery facilities via the downhole safety valve.
A wide range of different types of scale inhibitors are of course available. Generally the mechanism by which scale inhibition occurs is the same. It involves the scale inhibitor adsorbing to a scale nuclei or scale crystal in order to prevent its further growth. Whilst adsorbed on the surface, further crystal growth is impeded. The overall effect of scale inhibitors is therefore to slow down the rate of crystal growth. However, even at concentrations above the identified minimum inhibitor concentration (MIC) complete protection from scale precipitation will not be obtained.
DHCI has been used for a number of years and has been successful in many fields. On the other hand, however, DHCI has failed in a number of other fields. This is discussed in some detail in SPE article 154967 (Hustad, B. M., et al., Aberdeen 30-31 May 2012). Problems that have been encountered during DHCI include:                Significant scale growth in long flowlines due to long residence times and variations in temperature        Scale blockage of the chemical injection line and/or downhole safety valve        Gunking or plugging in gas lift operations due to the evaporation of solvent causing precipitation of solid scale inhibitor in the injection system        Corrosion of production equipment due to the acidity of certain scale inhibitors in combination with downhole conditions, especially high temperatures        Scale inhibitor freezing during transportation, especially in long flowlines        
Accordingly there is a need for an alternative, more efficient, method for controlling scale formation in a hydrocarbon producing system. Methods which are able to control scale formation when long (e.g. up to 50 km) flowlines are in use and/or wherein the flowlines encounter a wide range of temperature conditions are particularly desired.